The present invention relates to an apparatus and method for embossing a moving web of material, such as paper, to create a functional controlled degradation of the machine direction strength of the web while limiting degradation of the cross-machine direction strength of the web. In particular, the present invention relates to an apparatus and method for embossing a moving web using an embossing system having perforate embossing elements oriented to define perforating nips substantially oriented in the cross-machine direction to improve the flexibility, feel, bulk, and absorbency of the paper.
Embossing is the act of mechanically working a substrate to cause the substrate to conform under pressure to the depths and contours of a patterned embossing roll. Generally the web is passed between a pair of embossing rolls that, under pressure, form contours within the surface of the web. During an embossing process, the roll pattern is imparted onto the web at a certain pressure and/or penetration. In perforate embossing the embossing elements are configured such that at least a portion of the web located between the embossing elements is perforated. As used herein, generally, xe2x80x9cperforatedxe2x80x9d refers to the existence of either (1) a macro-scale through aperture in the web or (2) when a macro-scale through aperture does not exist, at least incipient tearing such as would increase the transmittivity of light through a small region of the web or would decrease the machine direction strength of a web by at least 15% for a given range of embossing depths.
Embossing is commonly used to modify the properties of a web to make a final product produced from that web more appealing to the consumer. For example, embossing a web can improve the softness, absorbency, and bulk of the final product. Embossing can also be used to impart an appealing pattern to a final product.
Embossing is carried out by passing a web between two or more embossing rolls, at least one of which carries the desired emboss pattern. Known embossing configurations include rigid-to-resilient embossing and rigid-to-rigid embossing.
In a rigid-to-resilient embossing system, a single or multi-ply substrate is passed through a nip formed between a roll whose substantially rigid surface contains the embossing pattern as a multiplicity of protuberances and/or depressions arranged in an aesthetically-pleasing manner, and a second roll, whose substantially resilient surface can be either smooth or also contain a multiplicity of protuberances and/or depressions which cooperate with the rigid surfaced patterned roll. Commonly, rigid rolls are formed with a steel body which is either directly engraved upon or which can contain a hard rubber-covered, or other suitable polymer, surface (directly coated or sleeved) upon which the embossing pattern is formed by any convenient method such as, for example, being laser engraved. The resilient roll may consist of a steel core provided with a resilient surface, such as being directly covered or sleeved with a resilient material such as rubber, or other suitable polymer. The rubber coating may be either smooth or engraved with a pattern. The pattern on the resilient roll may be either a mated or a non-mated pattern with respect to the pattern carried on the rigid roll.
In the rigid-to-rigid embossing process, a single-ply or multi-ply substrate is passed through a nip formed between two substantially rigid rolls. The surfaces of both rolls contain the pattern to be embossed as a multiplicity of protuberances and/or depressions arranged into an aesthetically-pleasing manner where the protuberances and/or depressions in the second roll cooperate with those patterned in the first rigid roll. The first rigid roll may be formed, for example, with a steel body which is either directly engraved upon or which can contain a hard rubber-covered, or other suitable polymer, surface (directly coated or sleeved) upon which the embossing pattern is engraved by any conventional method, such as by laser engraving. The second rigid roll can be formed with a steel body or can contain a hard rubber covered, or other suitable polymer, surface (directly coated or sleeved) upon which any convenient pattern, such as a matching or mated pattern, is conventionally engraved or laser-engraved. In perforate embossing, a rigid-to-rigid embossing system is typically used. However, a rigid-resilient configuration can also be used for perforate embossing.
When substantially rectangular embossing elements have been employed in perforate embossing, the embossing elements on the embossing rolls have generally been oriented so that the long direction axis, i.e., the major axis, of the elements is in the machine direction. That is, the major axis of the elements is oriented to correspond to the direction of the running web being embossed. These elements are referred to as machine direction elements. As a result, the elements produce perforations which extend primarily in the machine direction and undesirably decrease the strength of the web in the cross-machine direction. This orientation improves absorbency and softness, but can degrade, i.e., reduce the strength of, the web primarily in the cross-machine direction while less significantly degrading the strength of the web in the machine direction. As a result, the tensile strength of the web in the cross-machine direction is reduced relatively more, on a percentage basis, than that of the machine direction. In addition, the cross-machine direction strength of the base sheet is typically less than that of the machine direction strength. As a result, by embossing with machine direction elements, the cross-machine direction strength is even further weakened and, accordingly, because the finished product will fail in the weakest direction, the product will be more likely to fail when stressed in the cross-machine direction. Often, it is preferred that the web is xe2x80x9csquare,xe2x80x9d i.e., has a machine direction/cross-machine direction tensile ratio close to 1.0.
Cross-machine direction tensile strength can be associated with consumer preference for paper toweling. In particular, consumers prefer a strong towel, of which cross-machine direction and machine direction strength are two components. Because the un-embossed base sheet is typically much stronger in the machine direction than the cross-machine direction, a process is desired which results in both improved absorbency and softness without sustaining excessive losses in cross-machine direction tensile strength.
The present invention addresses at least the above described problem by providing at least two embossing rolls, wherein at least a portion of the elements are oriented to provide perforating nips which are substantially in the cross-machine direction and are configured to perforate the web, thereby allowing relatively greater degradation, i.e., a reduction of strength, of the web in the machine direction while preserving more of the cross-machine direction strength.
Further advantages of the invention will be set forth in part in the description which follows and in part will be apparent from the description or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
As embodied and broadly described herein, the invention includes an embossing system for embossing and perforating at least a portion of a web comprising a first embossing roll having embossing elements and at least a second embossing roll having embossing elements, wherein juxtaposition and engagement of the first and second embossing rolls define a plurality of perforate nips for embossing and perforating the web and wherein at least a predominate number of the embossing elements are configured so as to produce perforating nips which are substantially oriented in the cross-machine direction. In one embodiment, the invention further includes an embossing system wherein substantially all of the embossing elements of the first and second embossing rolls produce perforating nips which are substantially oriented in the cross-machine direction. Further, in a preferred embodiment, the cross-machine embossing elements are at an angle of 85 to 95xc2x0 from the machine direction.
In another embodiment, the invention includes an embossing system for embossing at least a portion of a web comprising a first embossing roll and at least a second embossing roll, wherein each of the first and second embossing rolls has at least one juxtaposable embossing element capable of producing a perforating nip substantially oriented in the cross-machine direction, thereby defining a cross-machine direction perforate nip between the cross-machine direction elements for embossing and perforating the web, and wherein at least a substantial portion of the cross-machine direction elements have at least the ends beveled.
In yet another embodiment, the invention includes an embossing system for embossing and perforating at least a portion of a web comprising a first embossing roll and at least a second embossing roll, wherein each of the first and second embossing rolls has at least one juxtaposable element capable of producing a perforating nip substantially oriented in the cross-machine direction, thereby defining a cross-machine direction perforate nip between the cross-machine direction elements for embossing and perforating the web, and wherein the cross-machine direction elements have sidewall angles, the angle between the sidewall and the radial direction on the cross-machine direction sides of the element, juxtaposed so as to be capable of producing a shear line, of less than about 20xc2x0. In one embodiment the cross-machine direction elements have cross-machine direction sidewall angles of less than about 17xc2x0. In another embodiment the cross-machine direction elements have cross-machine direction sidewall angles of less than about 14xc2x0. In a preferred embodiment, the cross-machine direction elements have cross-machine direction sidewall angles of less than 11xc2x0. In a further preferred embodiment the cross-machine direction elements have cross-machine direction sidewall angles of from about 7xc2x0 to 11xc2x0.
In yet another embodiment, the invention includes a method for embossing and perforating at least a portion of a web comprising providing a first embossing roll having embossing elements and providing at least a second embossing roll having embossing elements, wherein at least a predominate number of the embossing elements, when juxtaposed such that they are capable of producing perforate nips, are substantially oriented in the cross-machine direction and wherein the first and second embossing rolls define a perforate nip for embossing and perforating the web and passing the web between the first and second embossing rolls wherein the first and second embossing rolls are configured to result in an element clearance that will achieve a non-picking clearance while achieving at least a 15% reduction in the machine direction tensile strength of the web. We have found that it is desirable to exert special care to control the circumferential alignment of the two rolls to alleviate picking which may result from drift caused by local variances in roll diameter or gearing from the ideal.
In still yet another embodiment, the invention includes a method for reducing the tensile ratio of a web by embossing and perforating the web comprising passing a web through an embossing system, wherein the embossing system comprises a first embossing roll having embossing elements and at least a second embossing roll having embossing elements, wherein the first and second embossing rolls define a plurality of perforating nips for embossing and perforating the web and wherein at least a predominant number of the perforating nips which are substantially oriented in the cross-machine direction. In one embodiment, the invention further includes an embossing system wherein substantially all of the embossing elements of the first and second embossing rolls produce perforating nips which are substantially oriented in the cross-machine direction. Further, in a preferred embodiment, the cross-machine embossing elements are at an angle of 85-95xc2x0 from the machine direction.
In yet another embodiment, the invention includes a method for reducing the tensile ratio of a web by embossing and perforating the web comprising passing a web through an embossing system, wherein the embossing system comprises a first embossing roll and at least a second embossing roll, wherein each of the first and second embossing rolls has at least one juxtaposable embossing element capable of producing a perforating nip substantially oriented in the cross-machine direction, thereby defining a cross-machine direction perforate nip between the cross-machine direction elements for embossing and perforating the web and wherein at least a substantial portion of the cross-machine direction elements have at least the ends beveled.
In still yet another embodiment, the invention includes a method for reducing the tensile ratio of a web by embossing and perforating the web comprising, passing a web through an embossing system, wherein the embossing system comprises a first embossing roll and at least a second embossing roll, wherein each of the first and second embossing rolls has at least one juxtaposable embossing element capable of producing a perforating nip substantially oriented in the cross-machine direction, thereby defining a cross-machine direction perforate nip between the cross-machine direction elements for embossing and perforating the web and wherein the cross-machine direction elements have cross-machine direction sidewall angles of less than about 20xc2x0. In one embodiment the cross-machine direction elements have cross-machine direction sidewall angles of less than about 17xc2x0. In another embodiment the cross-machine direction elements have cross-machine direction sidewall angles of less than about 14xc2x0. It is preferred that the cross-machine direction elements have cross-machine direction sidewall angles of less than about 11xc2x0. It is further preferred that the cross-machine direction elements have cross-machine direction sidewall angles of from about 7xc2x0 to 11xc2x0.
In another embodiment, the invention includes a method for reducing the tensile ratio of a web by embossing and perforating the web comprising passing a web through an embossing system, wherein the embossing system comprises a first embossing roll having embossing elements and at least a second embossing roll having embossing elements, wherein the first and second embossing rolls define a perforate nip for embossing and perforating the web and wherein the first and second embossing rolls are configured to result in an element clearance that will achieve a non-picking clearance.
The invention further includes a perforate embossed web having a plurality of cross-machine direction oriented perforations wherein the embossed web has a tensile ratio of less than about 1.2. The invention further includes a perforate embossed web having a transluminance ratio (as defined hereinafter) of at least 1.005. Still further, the invention includes a wet-laid cellulosic perforate embossed web having perforate embossments extending predominately in the cross-machine direction.
Finally, the invention includes a method of embossing and perforating the web comprising passing a web through an embossing system, wherein the embossing system comprises a first embossing roll having embossing elements and at least a second embossing roll having embossing elements, wherein the first and second embossing rolls define a plurality of perforate nips for embossing and perforating the web, and wherein the tensile ratio of the web is reduced.
The accompanying drawings, which are incorporated herein and constitute a part of this specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention.